The fact that you do not see blood when you cough is reassuring, because this makes it unlikely that you have a serious infection or other source of bleeding
in your chest or lungs. There are several conditions that can cause bleeding with cough, such as a tumor in the lungs or a tuberculosis infection; however, generally in these conditions the amount of blood that is produced is sufficient that it is obvious with coughing.
However, it is still possible to 'taste' blood with cough related to several other conditions. This metallic taste might be because of microscopic amounts of blood from the irritation of the lining of the nose and throat from coughing or blowing your nose. It is also possible that the metallic blood taste is actually from chronic mucus congestion, which often will produce a similar taste to blood.
It sounds like these symptoms have been going on for a while, so I recommend that you go see your primary care doctor
for an evaluation. They will be able to help figure out where your cough is coming from - common causes include asthma
, nasal allergies, and sinus congestions - and recommend an appropriate treatment so that you can start feeling better.